The H-bridge
A few of you have asked how one goes about controlling the direction of the DC motor. You can not do this using the one transistor, one diode circuit that you have used from the SIK Guide. This simple circuit will only spin the motor in one direction but it does allow you to control the speed of the motor using PWM.If you wish to control both speed and direction you need to use a circuit that is referred to as and H-Bridge. The simplest H-bridge can be constructed using four transistors, four diodes and four resistors. The circuit is shown below is from the Robot Room website where you can read about the construction and use of the circuit
Once you have looked this over and understand what's happening, you may decide you want to build one and test it - your SIK only contains two transistors and two diodes, so to you will need to combine resources with a classmate.
You can also decide to take the "easy" route and obtain an IC (integrated circuit) that contains multiple H-Bridges. You can get these in multiple versions. The one shown in the image below is from Sparkfun. You can find numerous tutorials on how to use these devices. For example
